This invention pertains to microlithography (projection transfer of a pattern, defined on a reticle or mask, onto a sensitive substrate using an energy beam). Microlithography is a key technique used, for example, in the fabrication of microelectronic devices such as integrated circuits and displays. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as the energy beam. Even more specifically, the invention pertains to charged-particle-beam (CPB) microlithography performed using a divided (segmented) reticle.
Exposure schemes exploited by conventional charged-particle-beam (CPB) microlithographic exposure apparatus can be divided broadly into the following three types: (1) spot-beam exposure systems, (2) variably shaped beam exposure systems, and (3) block (cell) exposure systems. Each of these exposure schemes provides prospects of superior resolution of pattern elements compared to optical microlithography, but exhibit greatly reduced throughput compared to optical micro lithography.
The exposure schemes (1) and (2) perform exposure by tracing a pattern using a charged particle beam having a very small spot diameter (round transverse profile) or a very small rectangular transverse profile. Consequently, throughput obtained with these schemes is very low.
Block exposure was developed to improve throughput over that obtainable using schemes (1) and (2). Block exposure is especially useful for patterns in which a pattern unit (cell) is repeated many times, such as in a pattern containing a large number of identical memory cells. The pattern unit (typically measuring, e.g., 5 xcexcmxc3x975 xcexcm on the reticle) covers a larger portion of the overall pattern than is exposed at any one instant by schemes (1) or (2). The pattern unit is transferred onto respective locations on the wafer by individual respective xe2x80x9cshots.xe2x80x9d I.e., at each respective location on the wafer, the pattern unit is batch-exposed, thereby improving throughput. Unfortunately, this scheme is practical only for a few types of patterns (as noted above, the patterns typically are characterized by having a basic pattern unit or xe2x80x9ccellxe2x80x9d that is repeated many times in the pattern). However, essentially all such patterns also include non-repeated portions that cannot be formed on the wafer by projecting the cell. Rather, the non-repeated portions typically are exposed using the variable-shaped beam exposure scheme, so throughput is not increased as much as desired.
To improve throughput of CPB microlithography apparatus and methods, considerable research has focused on exposure schemes employing a xe2x80x9cdividedxe2x80x9d or xe2x80x9csegmentedxe2x80x9d reticle. In a divided-reticle scheme, the entire reticle pattern is divided into multiple subfields (each defining a respective portion of the overall pattern) that are exposed individually onto a suitable xe2x80x9csensitivexe2x80x9d substrate in an ordered manner (e.g., sequentially). On the reticle, each subfield can have dimensions of, for example, 1 mmxc3x971 mm, which is many times larger than a pattern unit used in the block exposure scheme. Hence, throughput using the divided reticle scheme is correspondingly greater.
Divided-reticle CPB microlithography is described further in connection with FIGS. 6, 7(a)-7(b), and 8. Turning first to FIG. 6, a profile of a typical substrate is shown (in this instance a semiconductor xe2x80x9cwaferxe2x80x9d). The substrate is rendered xe2x80x9csensitivexe2x80x9d by application of a coating of a xe2x80x9cresistxe2x80x9d that is reactive to exposure by a charged particle beam. Thus, the substrate can be imprinted with a latent image of a pattern defined on the reticle. As shown in the figure, the substrate includes a plurality of xe2x80x9cchipsxe2x80x9d or xe2x80x9cdies.xe2x80x9d Each chip is divided into a plurality of stripes (without intending to be limiting in any way, the figure shows, by way of example, four stripes a-d). Each stripe is divided into multiple subfields.
After completing exposure of a die, each subfield on the reticle has a counterpart subfield image in the die as formed on the substrate. The subfield images on the substrate typically are smaller than corresponding subfields on the reticle, usually by a factor termed a xe2x80x9cdemagnification ratio,xe2x80x9d such as 1/4 or 1/5, that is determined by the projection lens used to project the pattern from the reticle to the substrate. For example, if the demagnification ratio is 1/4, then a reticle subfield is four times larger than the corresponding image of the subfield on the substrate.
A portion of a divided reticle is shown in FIGS. 7(a)-7(b), depicting only one representative reticle subfield 100. FIG. 7(a) provides a plan view of the subfield 100 (with surrounding area), and FIG. 7(b) is an elevational section along the line Axe2x80x94A. For convenience in explanation, the size relationship of the subfield image on the substrate to the corresponding subfield on the reticle is not depicted accurately in the figure. The pattern element defined in the subfield 100 is denoted by the number 101. The pattern-defining portion 101 typically has a square profile with each side measuring 1 mm, for example. The pattern-defining portion 101 in this example defines a U-shaped pattern element 105. In this example, the pattern element 105 is defined by a corresponding aperture in a reticle membrane 104, thereby indicating that the depicted reticle is a xe2x80x9cstencilxe2x80x9d reticle. Surrounding the subfield 101 is an unpatterned skirt 102 bounded by a strut region 103. The strut region 103 has a width of 200 xcexcm, for example. Extending away from each strut region 103 is a respective strut 106. The strut 106 serves mainly to strengthen and provide rigidity to the reticle. The skirt 102, having an exemplary width of about 100 xcexcm, allows a certain positional tolerance for the beam illuminating the subfield 101. I.e., even if the beam experiences a limited amount of lateral positional displacement, the skirt 102 allows the beam nevertheless to illuminate the entire subfield 101 without impinging on a strut region 103. As shown in FIG. 7(b), the strut 106 is relatively thick in the Z-dimension. If the beam illuminating the reticle should impinge on a strut region 103 or strut 106, then significant heating of the reticle would result, which probably would cause undesirable reticle distortion.
It is not necessary that all subfields of a pattern be defined on a single reticle. The subfields alternatively can be distributed among multiple reticles.
Divided-reticle projection-exposure using a charged particle beam generally is performed in a manner as shown in FIG. 8, which depicts an exemplary exposure, within a stripe, of a row of subfields. During exposure, the reticle (mounted on a reticle stage) and substrate (mounted on a substrate stage) move synchronously in opposite directions relative to each other. The stage motions can be continuous xe2x80x9cscanningxe2x80x9d motions at fixed respective velocities that are determined mainly by the demagnification ratio of the projection lens used to project respective images of the subfields onto the substrate. During exposure of individual subfields, unpatterned regions (i.e., skirts 102 and strut regions 103) are not exposed. Hence, the ratio of substrate-stage velocity to reticle-stage velocity is not exactly equal to the demagnification ratio. I.e., to exclude imaging of skirts and strut regions, the velocity of the substrate stage relative to the reticle stage is slightly slower than would be dictated by the demagnification ratio. As the xe2x80x9cillumination beamxe2x80x9d illuminates a subfield on the reticle, an image of the illuminated subfield is projected, via a xe2x80x9cpatterned beamxe2x80x9d passing through a projection-optical system, onto a corresponding region on the substrate, thereby xe2x80x9cexposingxe2x80x9d the substrate with the image. During exposure of successive rows of subfields, the reticle stage and substrate stage move in opposite directions in one dimension (e.g., X-dimension). Meanwhile, the illumination beam and patterned beam are deflected in opposite directions in a second dimension (e.g., Y-dimension) to expose successive subfields in each row. Such deflections of the illumination beam and patterned beam are imparted by respective deflectors.
Thus, each stripe of the pattern is exposed row-by-row in a raster manner, and the pattern is exposed stripe-by-stripe to complete exposure of a single chip on the substrate.
Because a divided-reticle CPB microlithography apparatus performs exposure using the scheme summarized above (wherein each subfield is xe2x80x9cbatchxe2x80x9d-exposed and the subfields are exposed sequentially in a step-and-repeat or continuous scanning manner), throughput can be improved greatly over that obtainable using the three prior schemes listed above. However, increasing subfield size to improve throughput causes substantial problems such as xcex94blur (dispersion of blur) and increased distortion within subfield images. If xcex94blur within a subfield is sub-optimal, then the uniformity of pattern line widths within the subfield is degraded. If distortion within a subfield becomes excessive, then the connecting (stitching) accuracy of adjoining subfield images on the substrate, and the overlay accuracy of successive layers of the chip, become poor. This makes it difficult or impossible to fabricate acceptable microelectronic devices. As the density of microelectronic devices continues to increase, these problems tend to become increasingly difficult to accommodate and solve.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide charged-particle-beam (CPB) microlithography methods and apparatus that achieve improved xcex94blur and reduced distortion within individual subfields or other exposure units as projected from a segmented reticle onto the substrate. Another object is to achieve these objectives while maintaining acceptably high throughput.
According to a first aspect of the invention, methods are provided for performing CPB microlithography in which a region on a pattern-defining reticle, divided into multiple subfields, is illuminated with an illumination beam to produce an image of the illuminated region as projected on a sensitive substrate. A representative embodiment of such a method is directed specifically to methods for exposing the subfields. The subfields are subdivided into respective groups each containing multiple (at least two) subregions. An illumination beam is directed in sequence to each of the groups. At each group, the illumination beam is directed to expose the respective subregions in the group in a predetermined order before directing the illumination beam to a subsequent group. The subregions in each group can be exposed in the predetermined order that is identical from one group to the next in the sequence. Alternatively, the subregions in each group can be exposed in the predetermined order that is identical for every other group in the sequence.
The step of directing the illumination beam in sequence to each of the groups desirably is performed using a first deflector. The step of directing the illumination beam to expose the respective subregions in the group in a predetermined order before directing the illumination beam to a subsequent group desirably is performed using a second deflector. The first and second deflectors desirably are provided in an illumination-optical system of the apparatus.
According to another aspect of the invention, CPB microlithographic exposure apparatus are provided that expose a pattern, defined by a segmented reticle divided into multiple subfields each defining a respective portion of the pattern, onto a sensitive substrate using a charged particle beam so as to form respective transfer images of the subfields on the sensitive substrate. In a representative embodiment of such an apparatus, an illumination-optical system is situated and configured to direct a charged-particle illumination beam from a source to the segmented reticle, in which reticle each subfield comprises a respective group of multiple subregions. A first deflector is situated and configured to deflect the illumination beam from one group to the next on the reticle in a predetermined exposure sequence. A second deflector is situated and configured to deflect the illumination beam, within a group selected by the first deflector, to expose the respective subregions in the selected group in a predetermined order before the illumination beam is deflected to a subsequent group. A projection-optical system is situated and configured to form a respective image, on a sensitive substrate, of each subregion illuminated by the illumination beam. The apparatus desirably includes a controller connected to the first and second deflectors, wherein the deflector is configured to energize the first deflector in the predetermined exposure sequence and to energize the second deflector in the predetermined order.
The apparatus can include a third deflector situated between the reticle and the substrate. The third deflector is configured to direct, in synchrony with the second deflector, formation of the respective image on a corresponding predetermined location on the sensitive substrate.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.